Ionic liquids (ILs) are salts in which the cation and anion are poorly coordinated. At least one of the ionic components is organic and one of the ions has a delocalized charge. This prevents the formation of a stable crystal lattice, and results in such materials existing as liquids, often at room temperature, and at least, by definition, at less than 100° C. For example, sodium chloride, a typical ionic salt, has a melting point of about 800° C., whereas the ionic liquid N-methylimidazolium chloride has a melting point of about 75° C.
Ionic liquids typically comprise an organic cation, such as a substituted ammonium or a nitrogen-containing heterocycle, such as a substituted imidazolium, coupled with an inorganic anion. However, species have also been described wherein the cation and anion are organic. When the ionic liquid comprises at least one polymerizable group, it is a polymerizable ionic liquid (“PIL”).
Some of the features of ionic liquids that have caused the considerable research and development efforts in recent years include a broad liquid range and their high dissolving power—ionic liquids are excellent solvents for both organic and inorganic materials. Additionally, ionic liquids have high polarity, high thermal stability, high refractive indices, and high ionic conductivity. They are also non-flammable and have negligible vapor pressure. As a reaction medium, ionic liquids have been shown to accelerate many organic reactions including Diels-Alder cycloaddition reactions and alkylation reactions.
The combination of polymerizable functionality, room temperature organic liquid salt characteristics, high refractive index, high ionic conductivity, and negligible vapor pressure may enable their incorporation into new high value functional materials. Because of their low volatility, non-flammability, high solvency attributes, and potential ability to be recycled, ionic liquids have been promoted as environmentally safe or “green” solvents to replace conventional organic solvents (volatile organic compounds, VOCs).